Control over properties of the surface of an item, such as, for example, monitoring quality of the paper, metal and various film's surfaces, as well as control over thickness of the surface, is a key part during, for instance, process control and product quality control. One well-known and commonly utilized parameter for the surface quality characterization is a gloss degree. Optical measurement of the surface gloss degree is commonly used and well documented, for example, in ISO-standard 2813 “Paints and Varnishes—Determination of specular gloss if non-metallic paint films at 20° 60° and 80°”. At the time present, for example in steel industry, gloss degree is determined mainly in the laboratories in so called offline-mode, by using a separate point-like sensor. Determination of gloss parameters by using the point-like sensor is slow, and does not allow real-time tuning of the process, for example.
Also other systems for determining gloss of the surface are known from prior art. For example, in WO 01/20308 a solution is disclosed, wherein a light emitted by a light source is directed towards the measured surface so, that the light reflects from the surface to a detector. The detector, in turn, measures the intensity of the reflected light and determines a gloss of the surface on the basis of the intensity of the reflected light. In solution disclosed, a sensitivity of the detector can be changed. An instrument in turn, can be calibrated by a programmable constant value so that certain gloss units are given as the measurement result.
In addition, in EP 1 407 248 B1 a solution for surface gloss measurement is disclosed, wherein a light, emitted by a light source is collimated and split to two different beams, from which the first beam is directed via the first mirror towards the measured surface, and the second beam is directed through a prism to the first mirror, and, further, to the second mirror. The first beam is reflected from the measured surface to the second mirror and further from there to the detector. The second beam reflects, through the prism, to the second detector. In addition, a solution implies the use of a third detector in order to form a reference signal.
Still, as a prior art for measuring surface properties the document FI 119259 B is known, in which a solution is disclosed, wherein a light, emitted towards the surface, is decentralized, forming a spectrum so, that different wavelengths of the spectrum focus on different heights in the direction of the normal of the surface being measured. In the solution disclosed, on the basis of signal from the detector, an emission wavelength is determined at which the intensity of the optical radiation is the highest, and the location of the surface is determined based on the measured wavelength. In addition, according to the solution, the thickness of an object is determinable by measuring the location of upper- and lower surface planes.
However, known solutions have some drawbacks. For example, if the surface being measured is moving or trembling, an intensity of light, reflected from the surface, varies, regardless of a fact that gloss degree of the surface would be constant. It could also happen so, that focused light from the radiation source would not hit the proper spot on the surface being measured thereby the intensity of the reflected light could change. The reason for these drawbacks may be, for instance, that the distance and/or angle of trembling surface with respect to the detector are changing. Also the shape of the surface may vary, thereby affecting the intensity of reflected radiation, regardless of fact, that a gloss degree would stay unchanged. Therefore solutions disclosed in prior art do not necessarily lead to reliable results during measurements of moving surface properties, for example.